1. Field of the Invention
The present invention relates to an image reading apparatus, such as a digital copying machine or an image scanner, and an illumination drive method in the image reading apparatus. More particularly, the present invention relates to driving a fluorescent lamp at a high frequency.
2. Description of the Related Art
A variety of light sources are available for use with an image reading apparatus, such as a digital copying machine or a flat-bed scanner. Examples of such light sources include halogen lamps and fluorescent lamps of a hot or cold cathode type employing mercury vapor.
Halogen lamps have conventionally been used most frequently as light sources in digital copying machines. The halogen lamps are advantageous primarily in that a quantity of light and light distribution can be adjusted, and the quantity of light and hue are stable. The halogen lamps, however, are disadvantageous in that they require significant electric power because approximately 80% or more of the consumed power turns into heat. They are also susceptible to vibration because they employ filaments to emit light.
Flat-bed scanners mainly use fluorescent lamps because of their advantages of lower power consumption and longer service lives. Studies are being vigorously conducted to improve the efficiency of fluorescent lamps expected to be light sources for replacing halogen lamps in digital copying machines with higher productivity. The fluorescent lamps are starting to attract attention as light sources for replacing the halogen lamps in digital color copying machines that are required to provide high image quality.
Structurally, the fluorescent lamps come in several types; some of typical ones will be described below.
(1) Hot Cathode Fluorescent Lamp
This type has filaments releasing thermoelectrons at both ends of a fluorescent tube containing mercury vapor. The mercury is excited by released thermoelectrons and turned into visible light by a phosphor applied to the inside of the tube. The quantity of the thermoelectrons to be released is controlled by the electric current passed through the filaments thereby adjusting the quantity of light.
(2) Cold Cathode Fluorescent Lamp
In a cold cathode fluorescent lamp, a high voltage is applied to electrodes at both ends of a fluorescent tube to effect gas separation. This type of fluorescent lamp generally employs mercury vapor, and its name comes from the fact that it generates less heat than the hot cathode type. The cold cathode type features a service life that is longer than those of the hot cathode type by one order of magnitude or more because the electrodes are not exhausted.
(3) External-electrode Type Rare Gas Fluorescent Lamp
This type of lamp is represented by a xenon lamp having its fluorescent tube filled with a xenon gas. A high voltage is applied across the electrodes, which are provided so that they oppose each other outside the fluorescent tube, in order to excite xenon atoms which are turned into visible light by a phosphor. The type of components allows a longer service life; however, the use of xenon gas, which is more difficult to separate than mercury, requires that a higher voltage be applied and the external electrodes be provided with insulation. Generally, it is difficult to control a high voltage that is applied, so the quantity of light cannot be adjusted over a wide range.
The principle of luminescence applied to all types of lamps described above is that atoms sealed in the tubes are excited and converted into visible light by phosphors. Hence, the luminescence properties depend heavily on the characteristics of phosphors.
FIG. 9 shows emission spectrum characteristics of a typical white xenon lamp.
As shown in the graph, the white xenon lamp has a plurality of peaks, and is generally known as a three-wavelength type. Although the characteristics differ, depending on the manufacturer, most white fluorescent tubes in current use are of the three-wavelength type.
FIG. 10 illustrates differences in the luminescence properties of the three-wavelength type white fluorescent lamps and, more particularly, differences in RGB persistence characteristics. The RGB in the diagram may be considered equivalent to outputs of a CCD line sensor in a color image reading apparatus.
Light emitted by one lamp-on control maintains luminescence for a certain period of time while it weakens its luminous intensity according to the persistence characteristics of a luminescencer. As illustrated, the persistence characteristic of B (blue) is extremely shorter than that of R (red) or G (green). It is well accepted that the persistence characteristics of phosphors are such that R and G are about a few milliseconds (msec), and B is about a few microseconds (xcexcsec).
The differences in the persistence characteristics among R, G, and B have conventionally been presenting problems described below. The problems include irregular line cycles caused by variations in a luminescent integral value of each line, and color blurs on edges of documents in the vertical scanning direction.
The problem of the irregular line cycles will be first described with reference to FIG. 11.
In an example shown in FIG. 11, a lamp is turned on every 80 xcexcsec asynchronously with a video processing system of an apparatus, and image signals are captured every 250 xcexcsec. The persistence characteristics of the lamp, afterglow of R and G can be ignored relative to the lamp-on cycle of 80 xcexcsec, so that substantially uniform characteristics are observed. The afterglow of B is, however, short (typically a few xcexcsec), exhibiting the persistence characteristic relative to lamp-on signals as illustrated in the diagram. Therefore, a CCD output level of B varies on a line basis, leading to the above first problem, namely, irregular line cycles.
The problem of color blurs at the edges in the vertical scanning direction will now be described with reference to FIG. 12.
As illustrated, image signals are captured at 250-xcexcsec intervals, and the lamp is turned on once for each line in synchronization with the video processing system. Since the afterglows of the persistence characteristic of R and G are in the msec order, the influences by the afterglow of R and G can be ignored also under the condition shown in the diagram. The afterglow of B occurs in synchronization with the lamp-on signals, and extinguishes in a few xcexcsec from the moment a lamp-on signal turns OFF.
In the example shown in FIG. 12, an edge of a document image appears at a fourth line counted from a first line. If the document edge is located at a position where the lamp-on signal turns ON, then a balance of an RGB output of the CCD is ruined at the fourth line, resulting in the occurrence of a color blur at the edge.
The present invention is an attempt to solve the problems described above, and it is an object of the invention to provide an image reading apparatus capable of reducing output level fluctuations caused by differences in persistence characteristics of phosphors, and also reducing color blurs at edges of documents in a vertical scanning direction, and an illumination drive method for the same.
According to one aspect of the present invention, there is provided an image reading apparatus. This apparatus includes an illuminating unit adapted to illuminate a target. A photoelectric converting unit reads an image of target illuminated by the illuminating unit. A driving unit drives the illuminating unit such that the illuminating unit turns on a plurality of times during one storage period of time of the photoelectric converting unit. A lamp-on phase is changed based on a storage cycle of the photoelectric converting unit.
According to another aspect of the present invention, there is provided an illumination driving method for an image reading apparatus adapted to read, by photoelectric converting unit, a target illuminated by illuminating unit. A lamp is turned on a plurality of times during one storage period of time of the photoelectric converting unit and a lamp-on phase is changed based on a storage cycle of the photoelectric converting unit.
According to yet another aspect of the present invention, there is provided a storage medium storing a program for carrying out control such that a lamp is turned on a plurality of times during one storage period of time of photoelectric converting unit. A lamp-on phase is changed based on a storage cycle of the photoelectric converting unit when driving an illuminating device of an image of target reading apparatus for reading, by the photoelectric converting unit, a target illuminated by illuminating unit.